1. Field of the Invention
The present invention relates to an ignition apparatus for an internal combustion engine, and particularly to an ignition apparatus in which an actuating circuit for actuating one ignition plug is provided with two coil pairs, and multiple ignition is performed by generating a high secondary voltage at comparatively short time intervals.
2. Description of the Related Art
Japanese patent publication No. 5047247 (JP'247) discloses an ignition apparatus in which an actuating circuit for actuating one ignition plug is provided with two coil pairs, and ignition is performed by changing the used coil pair(s) according to electric discharge energy demand for the ignition plug. In this apparatus, ignition using one coil pair is performed when the electric discharge energy demand is comparatively low, and multiple ignition using two coil pairs in one cycle is performed in the boost-power ignition region where the electric discharge energy demand is comparatively high. Further, JP'247 also shows a control in which ignition is performed by alternately using one of the two coil pairs for every predetermined number of cycles.
According to the apparatus shown in JP'247, simultaneous discharge or alternate discharge using the two coil pairs is performed in the boost-power ignition region, as shown in FIG. 3 of JP'247. However in the simultaneous discharge, the discharge current of the ignition plug becomes comparatively large, which causes a problem that wear of the ignition plug by sputtering (collision of positive ions to the plug electrode) is readily occurred.
Further, in the alternate discharge, a temporary cut of the discharge current (a short period interruption of the discharge current) occurs, and the discharge is restarted immediately after the temporary cut. However, the discharge current becomes maximum at the time of starting the discharge (upon occurrence of the insulation breakdown). Accordingly, if frequency of starting the discharge becomes high, there is a problem that wear of the ignition plug is promoted by plug melting due to frequent start of the discharge. Therefore, in the above-described conventional apparatus, there is a room for improvement in the discharge control for suppressing wear of the ignition plug.